This invention relates to the recovery of copper from copper ores.
In processes for the recovery of copper from copper-containing ores in which copper is first extracted from the ores using an acid leach solution, followed by contacting the acid leach solution with organic solvent solutions containing oxime extractants, problems have been found with the processing of ores from certain locations such as ores from some areas of Chile. In particular, unacceptable degradation of the oximes has been found to occur, resulting in very high levels of oxime requirements per ton of copper produced from the ore, which of course results in a serious economic disadvantage.
Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term xe2x80x9caboutxe2x80x9d.
It has been discovered that the copper ores that cause degradation of oxime extractants are those in which the ores contain concentrations of nitrate ions, which are taken up by the aqueous acid leach solutions. When organic water-immiscible solvent solutions containing oxime extractants come in contact with the nitrate-containing acid leach solution, especially those containing nitrate levels above 10 g/l, nitration and/or hydrolysis of the oximes has been found to take place, resulting in large losses of oxime extractants. In addition, buildup of the nitrated oximes, which become loaded with copper as a copper complex and which cannot be effectively stripped under commercial operating conditions, causes viscosity of the organic solvent solutions to increase to unacceptable levels, resulting in such problems as loss of copper values, increased entrainment of the organic phase in the aqueous phase, and precipitation of the nitrated oxime copper complex from the organic phase. In some instances, the above problems have resulted in oxime extractant consumption of about eight times the oxime consumption when aqueous acid leach solutions which do not contain nitrate ions are processed in an otherwise identical manner.
In investigating the above problems it was discovered that
a) the presence of oxime extractant modifiers and/or additives significantly increased the rate of oxime degradation;
b) the presence of large quantities of aldoximes in the oxime extractants significantly increased the rate of oxime degradation;
c) when the pH of the acid leach solution was reduced to a pH range of from 2.25 to 3.1 oxime degradation was significantly reduced; and
d) an electromotive force of 550 mV or larger in the copper-pregnant acid leach solution in contact with the oxime-containing organic solvent solution significantly increased oxime degradation.
Accordingly, the present invention relates to the following process variants, used individually or in combination, for reducing oxime extractant degradation from contact with nitrate ion-containing copper pregnant acid leach solutions;
A) use of oxime extractants in water-immiscible organic solvent solutions wherein the solutions do not contain any modifiers or kinetic additives for the oxime extractants;
B) use of oxime extractants containing only ketoximes or a mixture of ketoximes and aldoximes in which the ketoxime:aldoxime molar ratio is less than 1:1.2, preferably less than 1:0.5, and more preferably 1:0.25 or less;
C) reducing the acidity of the acid leach solution to a pH in the range of from 2.25 to 3.1 prior to contact with the oxime-containing organic solvent solution;
D) reducing the electromotive force in the nitrate-containing copper-pregnant acid leach solution to less than 550 mV before contact with the organic solvent oxime extractant solution.
The solvent extraction process for extracting copper from copper ores typically involves the following steps:
1. Aqueous acid leaching of the copper ore using a strong acid to form an aqueous acid leach solution containing copper ions and often relatively small quantities of other metal ions. The aqueous leach acid solution dissolves salts of copper and other metals if present as it trickles through the ore. The metal values are usually leached with aqueous sulfuric acid, producing a leach solution having a pH of 0.99 to 2.0.
2. The copper-pregnant aqueous acid leach solution is mixed in tanks with an oxime extraction reagent which is dissolved in a water-immiscible organic solvent, e.g., a kerosene or other hydrocarbons. The reagent includes the oxime extractant which selectively forms a metal-extractant complex with the copper ions in preference to ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process.
3. The outlet of the mixer tanks continuously feeds to a large settling tank, where the organic solvent (organic phase), now containing the copper-extractant complex in solution, is separated from the depleted aqueous acid leach solution (aqueous phase). This part of the process is called phase separation. Usually, the process of extraction is repeated through two or more mixer/settler stages, in order to more completely extract the copper.
4. After extraction, the depleted aqueous acid leach solution (raffinate) is either discharged or recirculated to the ore body for further leaching.
5. The loaded organic phase containing the dissolved copper-extractant complex is fed to another set of mixer tanks, where it is mixed with an aqueous strip solution of concentrated sulfuric acid. The highly acid strip solution breaks apart the copper-extractant complex and permits the purified and concentrated copper to pass to the strip aqueous phase. This process of breaking the copper-extractant complex is called the stripping stage, and the stripping operation is repeated through two or more mixer-settler stages to more completely strip the copper from the organic phase.
6. As in the extraction process described above (step 2 and 3), the mixture is fed to another settler tank for phase separation.
7. From the stripping settler tank, the regenerated stripped organic phase is recycled to the extraction mixers to begin extraction again, and the copper is recovered from the strip aqueous phase, customarily by feeding the strip aqueous phase to an electrowinning tankhouse, where the copper metal values are deposited on plates by a process of electrodeposition.
8. After obtaining the copper values from the aqueous solution, the solution, known as spent electrolyte, is returned to the stripping mixers to begin stripping again.
The oxime extractants used in the above process are oxime extractants of the hydroxy aryl ketone oxime type or a mixture thereof with hydroxy aryl aldoximes. A general formula for such oximes is given on formula I shown below: 
in which A can be: 
(ii) Rxe2x80x2xe2x80x3or
(iii)H
where R and Rxe2x80x2 can be individually alike or different and are saturated aliphatic groups of 1-25 carbon atoms, ethylenically unsaturated aliphatic groups of 3-25 carbon atoms or ORxe2x80x3 where Rxe2x80x3 is a saturated or ethylenically unsaturated aliphatic group as defined; n is 0 or 1; a and b are each 0, 1, 2, 3, or 4, with the proviso that both are not 0 and the total number of carbon atoms in Ra and Rxe2x80x2b is from 3 to 25, Rxe2x80x2xe2x80x3 is a saturated aliphatic group of 1-25 carbon atoms or an ethylenically unsaturated aliphatic group of 3 to 25 carbon atoms, with the proviso that the total number of carbon atoms in Ra and Rxe2x80x2xe2x80x3 is from 3-25. Preferred compounds where A is (i) above are those in which a is 1, b is 0, R is a straight or branched chain alkyl group having from 7 to 12 carbon atoms and where R is attached in a position para to the hydroxyl group. Among those, the more preferred compounds are those wherein Rxe2x80x2xe2x80x3 is methyl and R and a are as designated. Compounds wherein n has a value of 0 (i.e. hydroxybenzophenone oxime compounds) can be prepared according to methods disclosed in Swanson U.S. Pat. Nos. 3,952,775 and 3,428,449. By reason of ready solubility in organic diluents commonly employed in solvent extraction and desirable properties of complexes of the compounds with copper, preferred benzophenone compounds are those having a single alkyl group of 7-12 carbon atoms in a position para to the hydroxyl group, in which the alkyl group is a mixture of isomers. Examples of such compounds are 2-hydroxy-5-nonylbenzophenone oxime and 2-hydroxy-5-dodecylbenzophenone oxime, which are obtained as mixtures of the isomeric forms when commercial nonylphenol and dodecylphenol are respectively employed in their synthesis.
Compounds wherein n has a value of 1 (i.e. hydroxy phenyl benzyl ketone oxime compounds) can be prepared according to methods described in Anderson U.S. Pat. No. 4,029,704. Preferred phenyl benzyl ketone oximes like the above noted benzophenone oximes are those having an isomeric mixture of 7 to 12 carbon alkyl groups as a single substituent on the ring para to the hydroxyl group. These preferred compounds are exemplified by the compound, 2-hydroxy-5-nonylphenyl benzyl ketone oxime, as manufactured from a commercial nonylphenol comprising a mixture of nonyl isomeric forms.
Compounds of the hydroxy phenyl alkyl ketone oxime type can be prepared according to the procedures disclosed in UK Patent 1,322,532, and are especially preferred for use herein. As noted with regard to the benzophenone and phenyl benzyl ketone compounds described above, the preferred compounds of this type are also those having an isomeric mixture of 7 to 12 carbon alkyl groups as a single substituent on the ring para to the hydroxyl group. Also preferred are those in which the Rxe2x80x2xe2x80x3 alkyl group is methyl. Illustrative of such preferred compounds where A is CH3 is 2-hydroxy-5-nonylphenyl methyl ketone oxime manufactured through the use of commercial nonylphenol.
Hydroxy aryl aldoxime extractants which can be employed in mixtures with ketoximes are those in which A is H. These hydroxy benzaldoximes, (also called xe2x80x9csalicylaldoximesxe2x80x9d), can be prepared according to methods described in Ackerley et al. U.S. Pat. Nos. 4,020,105 or 4,020,106 or by oximation of aldehydes prepared according to Beswick U.S. Pat. No. 4,085,146. Again preferred compounds are those having an isomeric mixture of isomeric 7 to 15 carbon alkyl groups as a single substituent para to the hydroxyl group, mixed alkyl isomeric forms of 2-hydroxy-5-heptyl benzaldoxime, 2-hydroxy-5-octyl benzaldoxime, 2-hydroxy -5-nonyl benzaldoxime and 2-hydroxy-5-dodecyl benzaldoxime are preferred, the most preferred for the purpose of the present invention where A is H is the dodecyl compound, i.e. 2-hydroxy-5-dodecyl benzaldoxime.
In one embodiment of the process of the present invention, the oxime extractant is either one or more ketoximes of formula (I) (i) or (I) (ii) or a mixture of one or more such ketoximes with one or more aldoximes of formula (I) (iii) above, in which the ketoxime:aldoxime molar ratio is less than 1:1.2, preferably less than 1:0.5, e.g. from 1:0.49 to 1:0.05, and more preferably is 1:0.25 or less.
The oxime extractants in the above process are typically used in prior processes in conjunction with modifiers such as one or more equilibrium modifiers, and kinetic active substances. Equilibrium modifiers include long chain aliphatic alcohols such an n-hexanol, 2-ethylhexanol, isodecanol, dodecanol, tridecanol, hexadecanol, and octadecanol; long chain alkylphenols such as heptylphenol, octylphenol, nonylphenol and dodecylphenol; organophosphorus compounds such as triloweralkyl (C4 to C8) phosphates, especially, tributyl phosphate and tri(2-ethylhexyl)phosphate; and either saturated or unsaturated aliphatic or aromatic-aliphatic esters containing from 10 to 30 carbon atoms, ketones, nitrates, ethers, amides, carbamates, carbonates, and the like. Kinetic active substances include xcex1,xcex2-hydroxy oximes described in Swanson, U.S. Pat. No. 3,224,873 and xcex1,xcex2-dioximes described in Koenders et al., U.S. Pat. No. 4,173,616.
In another embodiment of the invention, equilibrium modifiers and kinetic active substances are not used in the practice of the present invention, since they have been found to markedly increase the rate of oxime degradation from the nitrate ions.
The water-immiscible organic solvents used in the solvent extraction process of the invention are usually water-immiscible liquid hydrocarbon solvents. These include aliphatic and aromatic hydrocarbon diluents such as kerosene, benzene, toluene, xylene and the like. A choice of essentially water-immiscible hydrocarbon solvents or mixtures thereof will depend on factors, including the plant design of the solvent extraction plant, (mixer-settler units, extractors) and the like. The preferred solvents for use in the present invention are the aliphatic or aromatic hydrocarbons having flash points of 130xc2x0 Fahrenheit and higher, preferably at least 150xc2x0 and solubilities in water of less than 0.1% by weight. The solvents are essentially chemically inert. Representative commercially available solvents are Chevron(trademark) ion exchange solvent (available from Standard Oil of California) having a flash point of 195xc2x0 Fahrenheit; Escaid(trademark) 100 and 110 (available from Exxon-Europe) having a flash point of 180xc2x0 Fahrenheit; Norpar(trademark) 12 (available from Exxon-USA) with a flash point of 170xc2x0 Fahrenheit; Conoco(trademark) C 1214 (available from Conoco) with a flash point of 160xc2x0 Fahrenheit and C 170 exempt solvent having a flash point above 150xc2x0 Fahrenheit; and Aromatic 150 (an aromatic kerosene available from Exxon-USA having a flash point of 150xc2x0 Fahrenheit), and other various kerosene and petroleum fractions available from other oil companies, such as the ORFORM(trademark) SX series of solvent extraction diluents (available from Phillips 66: SX 1, 7, 11 and 12 each having a flash point above 150xc2x0 Fahrenheit varying up to 215xc2x0 Fahrenheit); and ESCAID(trademark) series of hydrocarbon diluents (available from Exxon: 100, 110, 115, 120, 200 and 300, each having a flash point above 150xc2x0 Fahrenheit; and EXXOL(trademark) D80 solvent (also available from Exxon and having a flash point above 150xc2x0 Fahrenheit).
In the process, the volume ratios of organic to aqueous (O:A) phase will vary widely since the contacting of any quantity of the oxime organic solution with the copper containing aqueous solution will result in the extraction of copper values into the organic phase. For commercial practicality however, the organic (O) to aqueous (A) phase ratios for extraction are preferably in the range of about 50:1 to 1:50. It is desirable to maintain an effective O:A ratio of about 1:1 in the mixer unit by recycle of one of the streams. In the stripping step, the organic:aqueous stripping medium phase will preferably be in the range of about 1:4 to 20:1. For practical purposes, the extracting and stripping are normally conducted at ambient temperatures and pressure although higher and lower temperatures and pressures are entirely operable although higher temperatures will increase oxime degradation. While the entire operation can be carried out as a batch operation, most advantageously the process is carried out continuously with the various streams or solutions being recycled to the various operations in the process for recovery of the copper metal, including the leaching, extraction and the stripping steps.
In the extraction process, the organic solvent solutions can contain the oxime extractant typically in an amount of about 2 to 15 weight/volume %.
After stripping of the copper values from the organic phase by the aqueous stripping solution and separation of the organic and aqueous stripping phase, the copper metal can be recovered by conventional recovery processes, including, but not limited to, precipitation and electrowinning. Electrowinning is typically the preferred means of recovery of the copper from solutions suitable for electrowinning, generally highly acidic aqueous solutions, such as a sulfuric acid solution containing greater than about 5 to about 200 g/l sulfuric acid, which is preferred as the aqueous acidic stripping solution to remove the copper values from the organic phase.
It is at this step, the stripping step, that the extraction reagent organic circuit phase which has degraded is removed from the extraction circuit after stripping for reoximation, prior to recovery of the metal from the stripping solution, with an optional scrub or wash step to remove any residual metals from the organic phase prior to reoximation and optional purification distillation where necessary or desirable.
Also, prior to stripping, it is not unusual to wash the organic phase, particularly where trace metals may be loaded on the organic extractant. One or more wash stages may accordingly be employed depending on any trace metals present, the amount of entrainment and the required purity of the final copper loaded stripping solution.
In a further embodiment of the invention, the acidity of the acid leach solution is reduced to a pH of from 2.25 to 3.1, preferably from 2.25 to 2.7 prior to contact with the oxime-containing organic solvent solution. The acidity reduction can conveniently be carried out by adding a base to the copper-containing acid leach solution, in which the base does not form an insoluble compound with the copper ions in the leach solution at a pH of 2.5 to 3.0. Sodium hydroxide is a preferred base for use herein. Alternatively, acidity reduction can be carried out by contacting the pregnant leach solution with an acid-consuming oxidic copper ore.
In another embodiment of the process of the invention, the electromotive force of the nitrate-containing copper-pregnant acid leach solution obtained in step 1. of the process is reduced to less than 550 mV, preferably about 500 mV or less, before contact with the oxime extractant-containing water-immiscible organic solvent solution in step 2. of the process. Various techniques can be employed to reduce the electromotive force. For example, the above leach solution can be passed over copper or iron metal, preferably scrap metal, or other divided forms of these metals, prior to step 2. of the process. Another method is to treat the above leach solution with sulfur dioxide prior to step 2. The above methods can also be combined, preferably by taking a portion of the leach solution, treating it with SO2, contacting the resulting leach solution with copper or iron scrap metal for a period of about 15, e.g. 5 to 30 minutes, and then returning this portion of the leach solution to the remainder of the leach solution. The quantity of SO2 added to the leach solution (based on the total quantity of leach solution) is not critical, and is generally in the range of 0.25 g. to 2 g. per liter of leach solution.